Surface modified nanoparticles

ABSTRACT

Surface-modified nanoparticles are produced by associating ligand interactive agents with the surface of a nanoparticle. The ligand interactive agents are bound to surface modifying ligands that are tailored to impart particular solubility and/or compatibility properties. The ligand interactive agents are crosslinked via a linking/crosslinking agent, such as hexamethoxymethylmelamine or a derivative thereof. The linking/crosslinking agent may provide a binding site for binding the surface modifying ligands to the ligand interactive agents.

BACKGROUND OF THE DISCLOSURE

A. Nanoparticles

There has been substantial interest in the preparation andcharacterization of compound semiconductors consisting of particles withdimensions in the order of 2-100 nm, often referred to as quantum dotsand/or nanoparticles. These studies have focused mainly on thesize-tunable electronic, optical and chemical properties ofnanoparticles. Semiconductor nanoparticles are gaining substantialinterest due to their applicability for commercial applications asdiverse as biological labeling, solar cells, catalysis, biologicalimaging, and light-emitting diodes.

Two fundamental factors, both related to the size of the individualsemiconductor nanoparticle, are primarily responsible for their uniqueproperties. The first is the large surface-to-volume ratio: as aparticle becomes smaller, the ratio of the number of surface atoms tothose in the interior increases. This leads to the surface propertiesplaying an important role in the overall properties of the material. Thesecond factor is that, with many materials including semiconductornanoparticles, the electronic properties of the material change withsize. Moreover, because of quantum confinement effects, the band gaptypically gradually becomes larger as the size of the nanoparticledecreases. This effect is a consequence of the confinement of an‘electron in a box,’ giving rise to discrete energy levels similar tothose observed in atoms and molecules, rather than a continuous band asobserved in the corresponding bulk semiconductor material. Semiconductornanoparticles tend to exhibit a narrow bandwidth emission that isdependent upon the particle size and composition of the nanoparticlematerial. The first excitonic transition (band gap) increases in energywith decreasing particle diameter.

Semiconductor nanoparticles of a single semiconductor material, referredto herein as “core nanoparticles,” along with an outer organicpassivating layer, tend to have relatively low quantum efficiencies dueto electron-hole recombination occurring at defects and dangling bondssituated on the nanoparticle surface that can lead to non-radiativeelectron-hole recombinations.

One method to eliminate defects and dangling bonds on the inorganicsurface of the quantum dot is to grow a second inorganic material,typically having a wider band-gap and small lattice mismatch to that ofthe core material, on the surface of the core particle, to produce a“core-shell” particle. Core-shell particles separate carriers confinedin the core from surface states that would otherwise act asnon-radiative recombination centers. One example is ZnS grown on thesurface of CdSe cores. Another approach is to prepare a core-multi shellstructure where the “electron-hole” pair is completely confined to asingle shell layer consisting of a few monolayers of a specific materialsuch as a quantum dot-quantum well structure. Here, the core is of awide bandgap material, followed by a thin shell of narrower bandgapmaterial, and capped with a further wide bandgap layer. An example isCdS/HgS/CdS grown using substitution of Hg for Cd on the surface of thecore nanocrystal to deposit just a few monolayers of HgS that is thenover grown by monolayers of CdS. The resulting structures exhibit clearconfinement of photo-excited carriers in the HgS layer.

The most studied and prepared semiconductor nanoparticles have beenII-VI materials, for example, ZnS, ZnSe, CdS, CdSe, and CdTe, as well ascore-shell and core-multi shell structures incorporating thesematerials. Other semiconductor nanoparticles that have generatedconsiderable interest include nanoparticles incorporating III-V andIV-VI materials, such as GaN, GaP, GaAs, InP, and InAs. Methods ofsynthesizing core and core-shell nanoparticles are disclosed, forexample, in co-owned U.S. Pat. Nos. 6,379,635, 7,803,423, 7,588,828,7,867,556, and 7,867,557. The contents of each of the forgoing patentsare hereby incorporated by reference, in their entirety.

B. Surface Modification

Many applications of nanoparticles require that the semiconductornanoparticle be compatible with a particular medium. For example, somebiological applications such as fluorescence labeling, in vivo imagingand therapeutics require that the nanoparticles be compatible with anaqueous environment. For other applications, it is desirable that thenanoparticles be dispersible in an organic medium such as aromaticcompounds, alcohols, esters, or ketones. For example, ink formulationscontaining semiconductor nanoparticles dispersed in an organicdispersant are of interest for fabricating thin films of semiconductormaterials for photovoltaic (PV) devices.

A particularly attractive potential field of application forsemiconductor nanoparticle is in the development of next generationlight-emitting diodes (LEDs). LEDs are becoming increasingly important,in for example, automobile lighting, traffic signals, general lighting,and liquid crystal display (LCD) backlighting and display screens.Nanoparticle-based light-emitting devices have been made by embeddingsemiconductor nanoparticles in an optically clear (or sufficientlytransparent) LED encapsulation medium, typically a silicone or anacrylate, which is then placed on top of a solid-state LED. The use ofsemiconductor nanoparticles potentially has significant advantages overthe use of the more conventional phosphors. For example, semiconductornanoparticles provide the ability to tune the emission wavelength of aLED. Semiconductor nanoparticles also have strong absorption propertiesand low scattering when the nanoparticles are well dispersed in amedium. The nanoparticles may be incorporated into an LED encapsulatingmaterial. It is important that the nanoparticles be well dispersed inthe encapsulating material to prevent loss of quantum efficiency.Methods developed to date are problematic because the nanoparticles tendto agglomerate when formulated into LED encapsulants, thereby reducingthe optical performance of the nanoparticles. Moreover, even after thenanoparticles have been incorporated into the LED encapsulant, oxygencan still migrate through the encapsulant to the surfaces of thenanoparticles, which can lead to photo-oxidation and, as a result, adrop in quantum yield (QY).

A nanoparticle's compatibility with a medium as well as thenanoparticle's susceptibility to agglomeration, photo-oxidation and/orquenching, is mediated largely by the surface composition of thenanoparticle. The coordination about the final inorganic surface atomsin any core, core-shell or core-multi shell nanoparticle is incomplete,with highly reactive “dangling bonds” on the surface, which can lead toparticle agglomeration. This problem is overcome by passivating(capping) the “bare” surface atoms with protecting organic groups,referred to herein as capping ligands or a capping agent. The capping orpassivating of particles not only prevents particle agglomeration fromoccurring. The capping ligand also protects the particle from itssurrounding chemical environment and provides electronic stabilization(passivation) to the particles, in the case of core material. Thecapping ligand is usually a Lewis base bound to surface metal atoms ofthe outer most inorganic layer of the particle. The nature of thecapping ligand largely determines the compatibility of the nanoparticlewith a particular medium. These capping ligand are usually hydrophobic(for example, alkyl thiols, fatty acids, alkyl phosphines, alkylphosphine oxides, and the like). Thus, the nanoparticles are typicallydispersed in hydrophobic solvents, such as toluene, following synthesisand isolation of the nanoparticles. Such capped nanoparticles aretypically not dispersible in more polar media.

The most widely used procedure to modify the surface of nanoparticles isknown as ligand exchange. Lipophilic ligand molecules that coordinate tothe surface of the nanoparticle during core synthesis and/or shellingprocedures may subsequently be exchanged with a polar/charged ligandcompound. An alternative surface modification strategy intercalatespolar/charged molecules or polymer molecules with the ligand moleculesthat are already coordinated to the surface of the nanoparticle. Currentligand exchange and intercalation procedures may render the nanoparticlemore compatible with aqueous media but usually result in materials oflower quantum yield (QY) and/or substantially larger size than thecorresponding unmodified nanoparticle.

Thus, there is a need in the art for nanoparticles that are compatiblewith a variety of media and for techniques for modifying the surface ofnanoparticles to render desired compatibility while maintaining theintegrity and photophysical properties of the nanoparticle.

The subject matter of the present disclosure is directed to overcoming,or at least reducing the effects of, one or more of the problems setforth above.

SUMMARY OF THE DISCLOSURE

The present disclosure provides methods for modifying the surface ofnanoparticles and of producing surface-functionalized nanoparticles. Ingeneral, the method includes associating a first type of molecule,referred to herein as a ligand interactive agent, with the surface ofthe nanoparticle. The ligand interactive agent is then reacted with alinking/crosslinking agent. The linking/crosslinking agent can serve twopurposes: (1) it provides crosslinking between the molecules of theligand interactive agent (and potentially also crosslinking betweenother ligands on the nanoparticle surface), and (2) it provides ananchor point for one or more surface modifying ligands.

The ligand interactive agent may associate with the surface of thenanoparticle via one or more of several different modes. For example,the ligand interactive agent may associate with the surface of thenanoparticle by intercalating with ligands, such as capping ligands,already present on the nanoparticle surface. The ligand interactiveagent may associate with the nanoparticle surface via ligand exchangewith such existing ligands. The ligand interactive agent may, or maynot, include one or more functional groups that have affinity for thenanoparticle surface. One or more of these modes of interaction betweenthe ligand interactive agent and the nanoparticle surface may beoperative at a given time.

The ligand interactive agent includes one or more functional groups thatinteract with a linking/crosslinking agent. A linking/crosslinking agentprovides crosslinking between molecules of the ligand interactive agenton the nanoparticle surface. Thus, the linking/crosslinking agentbecomes incorporated into the ligand shell of the nanoparticle. Thelinking/crosslinking agent may have specific affinity for, or reactivitywith, functional groups of the ligand interactive agent and may bridgebetween such functional groups. The linking/crosslinking agent may bemulti-dentate and may bridge between two, three, or more ligands on thenanoparticles surface.

Crosslinking may increase the stability and robustness of the ligandshell of the nanoparticle. As a result, the nanoparticle may be lesssusceptible to degradation, quenching, photo-bleaching, and the like.

An initiator or catalyst may be used to initiate or facilitatecrosslinking. The initiator or catalysts may be a chemical initiator,such as an acid, for example. The initiator may be a photo-initiator.

The linking/crosslinking agent may also serve as an attachment point fora surface modifying ligand. The surface modifying ligand may incorporatefunctionality that modifies the compatibility of the surface-modifiednanoparticle with particular solvents or media. For example, the surfacemodifying ligand may incorporate polar groups, increasing thecompatibility of the surface-modified nanoparticle with polar solvents,such as water, alcohols, ketones, ink resins, epoxy resins, and polaracrylate resins. As another example, the surface modifying ligand maycomprise a silicone, or the like, increasing the compatibility of thesurface modified nanoparticle with a silicone matrix. Particular ligandinteractive agents, linking/crosslinking agents, and surface modifyingligands are discussed in more detail below.

According to one embodiment, the ligand interactive agent is firstassociated with the surface of the nanoparticle. The nanoparticle isthen reacted with the linking/crosslinking agent and the surfacemodifying agent to effect crosslinking and binding of the surfacemodifying agent to the nanoparticle.

According to another embodiment, the surface modifying ligand ispre-associated with the ligand interactive agent. The nanoparticle isexposed to the ligand interactive agent/surface modifying ligandcombination which associates to the surface of the nanoparticle. Thenanoparticle is then exposed to the linking/crosslinking agent to effectcrosslinking.

The foregoing summary is not intended to summarize each potentialembodiment or every aspect of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a method of modifying the surfaceof a nanoparticle.

FIG. 2 is a schematic illustration of a ligand interactive agent.

FIG. 3 illustrates a method of modifying the surface of a nanoparticleusing isopropyl myristate as a ligand interactive agent, HMMM as alinking/crosslinking agent, and PEG as a surface modifying ligand.

FIG. 4 illustrates a method of modifying the surface of a nanoparticlewith a PEG-modified myristate surface modifying ligand.

FIG. 5A shows the fluorescence spectrum of silicone-compatiblenanoparticles suspended in PDMS.

FIG. 5B shows the fluorescence spectrum of unmodified nanoparticlessuspended in PDMS.

FIG. 6A shows the fluorescence spectrum of epoxy-compatiblenanoparticles suspended in epoxy resin.

FIG. 6B shows the fluorescence spectrum of unmodified nanoparticlessuspended in epoxy resin.

FIG. 7 shows a fluorescence spectrum of water soluble nanoparticles inwater.

FIG. 8A shows the emission spectrum of an LED incorporatingepoxy-compatible nanoparticles suspended in an epoxy encapsulant.

FIG. 8B shows the emission spectrum of an LED incorporating unmodifiednanoparticles encapsulated in acrylate beads suspended in epoxy.

FIG. 9A shows stability measurements of an LED incorporatingepoxy-compatible nanoparticles suspended in an epoxy encapsulant.

FIG. 9B shows stability measurements of an LED incorporating unmodifiednanoparticles encapsulated in acrylate beads suspended in epoxy.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an embodiment of a method of producingsurface modified nanoparticles. A nanoparticle 100 includes a shell oforganic ligands 101 associated with the surface of the nanoparticle. Theinstant disclosure is not limited to any particular type ofnanoparticle. Nanoparticles of metal oxides (for example, iron oxides,magnetic nanoparticles, titanium oxides, zinc oxide, zirconium oxide,aluminum oxide), gold nanoparticles and silver nanoparticles can be alltreated and surface-modified using the methods described herein. Inpreferred embodiments, the nanoparticle may include a semiconductormaterial, preferably a luminescent semiconductor material. Thesemiconductor material may incorporate ions from any one or more ofgroups 2 to 16 of the periodic table, and may include binary, ternaryand quaternary materials, that is, materials incorporating two, three orfour different ions respectively. By way of example, the nanoparticlemay incorporate a semiconductor material, such as, but not limited to,CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AlP, AlS, AlAs, AlSb,GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge and combinations thereof.According to various embodiments, nanoparticles may have diameters ofless than around 100 nm, less than around 50 nm, less than around 20 nm,less than around 15 nm and/or may be in the range of around 2 to 10 nmin diameter.

Nanoparticles that include a single semiconductor material, e.g., CdS,CdSe, ZnS, ZnSe, InP, GaN, etc. may have relatively low quantumefficiencies because of non-radiative electron-hole recombination thatoccurs at defects and dangling bonds at the surface of thenanoparticles. In order to at least partially address these issues, thenanoparticle cores may be at least partially coated with one or morelayers (also referred to herein as “shells”) of a material differentthan that of the core, for example a different semiconductor materialthan that of the “core.” The material included in the, or each, shellmay incorporate ions from any one or more of groups 2 to 16 of theperiodic table. When a nanoparticle has two or more shells, each shellmay be formed of a different material. In an exemplary core/shellmaterial, the core is formed from one of the materials specified aboveand the shell includes a semiconductor material of larger band-gapenergy and similar lattice dimensions as the core material. Exemplaryshell materials include, but are not limited to, ZnS, ZnO, MgS, MgSe,MgTe and GaN. An exemplary multi-shell nanoparticle is InP/ZnS/ZnO. Theconfinement of charge carriers within the core and away from surfacestates provides nanoparticles of greater stability and higher quantumyield.

While the disclosed methods are not limited to any particularnanoparticle material, an advantage of the disclosed methods is that themethods can be used to modify the surface of cadmium-free nanoparticles,that is, nanoparticles comprising materials that do not contain cadmium.It has been found that it is particularly difficult to modify thesurface of cadmium-free nanoparticles. Cadmium-free nanoparticlesreadily degrade when prior art methods, such as prior art ligandexchange methods, are used to modify the surface of such cadmium-freenanoparticles. For example, attempts to modify the surface ofcadmium-free nanoparticles have been observed to cause a significantdecrease in the luminescence quantum yield (QY) of such nanoparticles.The disclosed methods, on the other hand, provide surface-modifiedcadmium-free nanoparticles with high QY. For example, the disclosedmethods have resulted in cadmium-free nanoparticles that are dispersiblein water and which have QY greater than about 20%, greater than about25%, greater than about 30%, greater than about 35%, and greater thanabout 40%. Examples of cadmium free nanoparticles include nanoparticlescomprising semiconductor materials, e.g., ZnS, ZnSe, ZnTe, InP, InAs,InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, andparticularly, nanoparticles comprising cores of one of these materialsand one or more shells of another of these materials.

Typically, as a result of the core and/or shelling procedures employedto produce the core, core/shell or core/multishell nanoparticles, thenanoparticles are at least partially coated with a surface bindingligand 101, such as myristic acid, hexadecylamine and/ortrioctylphosphineoxide. Such ligands are typically derived from thesolvent in which the core and/or shelling procedures were carried out.While ligands 101 of this type can increase the stability of thenanoparticles in non-polar media, provide electronic stabilizationand/or negate undesirable nanoparticle agglomeration, as mentionedpreviously, such ligands typically prevent the nanoparticles from stablydispersing or dissolving in more polar media, such as aqueous solvents.

As a first step of modifying nanoparticle 100, the nanoparticle isexposed to ligand interactive agent 102 to effect the association ofligand interactive agent 102 and the surface of nanoparticle 100. Aschematic of ligand interactive agent 102 is shown in more detail inFIG. 2. Ligand interactive agent can comprise a chain portion 103 and afunctional group 104 having a specific affinity for, or reactivity with,a linking/crosslinking agent, as described below. Examples of suchfunctional groups 104 include nucleophiles such as thio groups, hydroxylgroups, carboxamide groups, ester groups, and a carboxyl groups. Anester is an example of such a functional group 104. Chain portion 103may be, for example, an alkane chain. Ligand interactive agent 102 may,or may not, also comprise a moiety 105 having an affinity for thesurface of a nanoparticle. Examples of such moieties 105 include thiols,amines, carboxylic groups, and phosphines. If ligand interactive group102 does not comprise such a moiety 105, ligand interactive group canassociate with the surface of nanoparticle 100 by intercalating withcapping ligands 101 (see FIG. 1). Examples of ligand interactive agents102 include C₈₋₂₀ fatty acids and esters thereof, such as isopropylmyristate.

Referring back to FIG. 1, it should be noted that ligand interactiveagent 102 may be associated with nanoparticle 100 simply as a result ofthe processes used for the synthesis of the nanoparticle, obviating theneed to expose nanoparticle 100 to additional amounts of ligandinteractive agent 102. In such case, there may be no need to associatefurther ligand interactive agent with the nanoparticle. Alternatively,or in addition, nanoparticle 100 may be exposed to ligand interactiveagent 102 after nanoparticle 100 is synthesized and isolated. Forexample, nanoparticle 100 may be incubated in a solution containingligand interactive agent 102 for a period of time. Such incubation, or aportion of the incubation period, may be at an elevated temperature tofacilitate association of ligand interactive agent 102 with the surfaceof nanoparticle 100. Associating ligand interactive agent 102 withnanoparticle 100 yields ligand interactive agent-nanoparticleassociation complex 110.

Following association of ligand interactive agent 102 with the surfaceof nanoparticle 100, the nanoparticle is exposed to linking/crosslinkingagent 106 and surface modifying ligand 107. Linking/crosslinking agent106 includes functional groups having specific affinity for groups 104of ligand interactive agent 102. Linking/crosslinking agent 106 also hasspecific reactivity with surface modifying ligand 107. Thus,linking/crosslinking agent 106 may serve to crosslink the ligand shellof nanoparticle 100 and also may serve to bind surface modifying ligand107 to the surface of nanoparticle 100.

Ligand interactive agent-nanoparticle association complex 110 can beexposed to linking/crosslinking agent 106 and surface modifying ligand107 sequentially. For example, nanoparticle 100 (including 102) might beexposed to linking/crosslinking agent 106 for a period of time to effectcrosslinking, and then subsequently exposed to surface modifying ligand107 to incorporate 107 into the ligand shell of nanoparticle 100.Alternatively, nanoparticle 100 may be exposed to a mixture of 106 and107, effecting crosslinking and incorporating surface modifying ligandin a single step.

Examples of suitable linking/crosslinking agents include any agent thatwill crosslink molecules of ligand interactive agent 102 and provide abinding site for surface modifying ligand 107. Particularly suitablelinking/crosslinking agents 106 comprise melamine-based compounds:

A particularly suitable melamine-based linking/crosslinking agent ishexamethoxymethylmelamine (HMMM):

HMMM is commercially available from Cytec Industries, Inc. (WestPaterson, N.J.) as CYMEL303. HMMM can react in an acid-catalyzedreaction to crosslink various functional groups, such as amides,carboxyl groups, hydroxyl groups, and thiols. In the presence of strongacid, HMMM crosslinks thiol-containing compounds at temperatures aboveabout 75° C. and crosslinks carboxyl- or amide-containing compounds attemperatures above about 130° C. These temperatures are not intended tobe limiting; lower temperatures, such as about 120° C., may result incrosslinking at a slower rate. An embodiment disclosed herein is acomposition comprising a nanoparticle and a melamine compound, such asHMMM. The composition may comprise a polar solvent. The composition maybe an ink formulation.

The presence of a strong proton acid is typically needed to catalyzecrosslinking with HMMM. The most active catalysts are those with thelowest pKa values. Examples of catalysts include mineral acids,p-toluene sulfonic acid, dinonylnapthalene disulfonic acid,dodecylbenzene sulfonic acid, oxalic acid, maleic acid, hexamic acid,phosphoric acid, alkyl phosphate ester, phthalic acid, acrylic acid, andsalicylic acid.

Referring back to FIG. 1, surface modifying ligand 107 is associatedwith nanoparticle 100 by binding to ligand interactive agent 102.Surface modifying ligand 107 can modify the compatibility of thenanoparticle with a particular solvent or media. For example,associating surface modifying ligand 107 with nanoparticle 100 mayrender nanoparticle 100 soluble, or at least more compatible, withaqueous solvents. Examples of such surface modifying ligands includepolyethers, such as polyethylene glycols. One example of a surfacemodifying ligand 107 is hydroxyl-terminated polyethylene glycol. Othersurface modifying ligands can be selected to impart compatibility withother media or solvents. For example, a silicone-based surface modifyingligand, such as polydimethysiloxane (PDMS) can be used as a surfacemodifying ligand to impart compatibility of the nanoparticle withsilicone resins and polycarbonate resins. As another example,guaifenesin can be used to impart compatibility with polar solvents andpolar acrylates, such as trimethylopropane trimethacrylate (TMPTM).

An embodiment as illustrated in FIG. 1 can be summarized as follows:Nanoparticle 100, incorporating capping ligand 101 is incubated in anappropriate solvent with ligand interactive agent 102 to effectassociation of 102 with the surface of nanoparticle 100.Linking/crosslinking agent 106, surface modifying ligand 107, and aninitiator or catalyst are added and the entire mixture is heatedtogether at a time and temperature sufficient to effect crosslinking andassociation of surface modifying ligand into the ligand shell ofnanoparticle 100.

FIG. 3 illustrates an embodiment wherein nanoparticle 300, includingcapping ligand 301 is exposed to isopropyl myristate as a ligandinteractive agent 302. According to the embodiment illustrated in FIG.3, the isopropyl myristate associates with the surface of thenanoparticle 300 by intercalating with the capping ligand. Suchintercalation can be effected by incubating the nanoparticle and theisopropyl myristate in a solvent, such as toluene for a period of timeranging from several minutes to several hours. According to oneembodiment, the nanoparticle and isopropyl myristate are heated intoluene to about 50-60° C. for about 5 minutes and then left at roomtemperature overnight. According to one embodiment, about 200 mg ofnanoparticles can be incubated with about 100 microliters of isopropylmyristate.

According to the embodiment illustrated in FIG. 3, HMMM is provided as alinking/crosslinking agent 306, salicylic acid as a catalyst 308, andmonomethoxy polyethylene oxide (mPEG) as surface modifying ligand 307. Amixture of HMMM, salicylic acid, and mPEG in toluene can be added to thenanoparticle mixture and heated to about 140° C. for a period of timeranging from about several minutes to several hours to yieldPEG-modified nanoparticle 309.

The embodiment illustrated in FIG. 3 results in a PEG-modifiednanoparticle that is compatible with an aqueous dispersant. The surfacemodifying ligand can be tailored to provide compatibility with othermedia. As mentioned above, a silicone-based surface modifying ligand,such as polydimethysiloxane (PDMS) can be used as a surface modifyingligand to impart compatibility of the nanoparticle with silicone resinsand polycarbonate resins. As another example, guaifenesin can be used toimpart compatibility with polar solvents and polar acrylates, such astrimethylopropane trimethacrylate (TMPTM). Generally, any surfacemodifying ligand that is reactive with HMMM and soluble in toluene canbe used in the embodiment illustrated in FIG. 3.

FIG. 4 illustrates an alternative embodiment wherein nanoparticle 400,including capping ligand 401, is treated with a surface modifying ligand402 that includes a functional group (an ester group) that is capable ofreacting with HMMM linking/crosslinking agent 406. Surface modifyingligand 402 is a myristate-based ligand that includes a functional group(PEG-OCH₃) that imparts water solubility to nanoparticle 400. Surfacemodifying ligand 402 may also include a functional group (denoted “X” inthe embodiment illustrated in FIG. 4) that has a specific affinity forthe surface of nanoparticle 400. Examples of such functional groupsinclude thiols and carboxylic groups.

Once surface modifying ligand is associated with the surface ofnanoparticle 400, nanoparticle 400 is then reacted withlinking/crosslinking agent 406 and catalyst 408 to effect crosslinkingbetween surface modifying ligands 402. HMMM is the linking/crosslinkingagent 406 and salicylic acid is the catalyst 408 in the embodimentillustrated in FIG. 4. Crosslinking the surface modifying ligandsincreases the stability of the ligand shell of the surface-modifiednanoparticle 409.

Examples

1. Silicone-Compatible Nanoparticles

Cadmium free quantum dot nanoparticles (CFQD) (InP/ZnS) (200 mg) withred emission at 608 nm was dispersed in toluene (1 mL) with isopropylmyristate (100 microliters). The mixture was heated at 50° C. for about1-2 minutes then slowly shaken for 15 hours at room temperature. Atoluene solution (4 mL) of HMMM (Cymel 303) (400 mg), monohydroxypolydimethyl siloxane (MW 5 kD) (200 mg), and p-toluene sulfonic acid(70 mg) was added to the nanoparticle dispersion. The mixture wasdegased and refluxed at 140° C. for 4 hours while stirring at 300 rpmwith a magnetic stirrer. During the first hour a stream of nitrogen waspassed through the flask to ensure the removal of volatile byproductsgenerated by the reaction of HMMM with nucleophiles. The mixture wasallowed to cool to room temperature and stored under inert gas. Thesurface-modified nanoparticles showed little or no loss in fluorescencequantum yield and no change in the emission peak or full width at halfmax (FWHM) value, compared to unmodified nanoparticles. Thesurface-modified nanoparticles dispersed well in PDMS polymers ofvariable molecular weight (from 10 to 1000 kD) and remained dispersedeven after removing residual toluene. In contrast, the sameconcentration of unmodified nanoparticles dispersed in PDMS aggregatedand separated out of the host silicone.

The films were prepared as follows: nanoparticles (6 mg) dispersed intoluene (˜200 microliters) were mixed well with of PDMS resin (1 g)using a spatula. The mixture was vigorously degased under vacuum forseveral hours to remove toluene. The mixture then was mounted on a glassslide to form a film.

FIGS. 5A and 5B illustrate fluorescence spectra of surface modifiednanoparticles and unmodified nanoparticles suspended in PDMS,respectively. For each of FIGS. 5A and 5B, four measurements wereperformed: one measurement of a blank sample with internal standard onlyand three measurements of the nanoparticles suspended in PDMS. Thefluorescence quantum yield of the surface-modified nanoparticles(QY=59%) is greater than that of the unmodified nanoparticles (QY=56%).The quantum yield of the unmodified nanoparticles is decreased due toextensive aggregation and reabsorption effects.

2. Epoxy-Compatible Nanoparticles.

Cadmium free quantum dot nanoparticles (CFQD) (InP/ZnS) (200 mg) withgreen emission at 525 nm was dispersed in toluene (1 mL) with isopropylmyristate (100 microliters). The mixture was heated at 50° C. for about1-2 minutes then slowly shaken for 15 hours at room temperature. Atoluene solution (4 mL) of HMMM (Cymel 303) (400 mg), trimethylolpropanetriglycidyl ether (200 mg) and salicylic acid (70 mg) was added to thenanoparticle dispersion. The mixture was degased and refluxed at 140° C.for 4 hours while stirring at 300 rpm with a magnetic stirrer. Duringthe first hour a stream of nitrogen was passed through the flask toensure the removal of volatile byproducts generated by the reaction ofHMMM with nucleophiles. The mixture was allowed to cool to roomtemperature and stored under inert gas. The surface-modifiednanoparticles showed little or no loss in fluorescence quantum yield andno change in the emission peak or full width at half max (FWHM) value,compared to unmodified nanoparticles. The surface-modified nanoparticlesdispersed well in epoxide polymers of variable molecular weight andremained dispersed even after removing residual toluene. In contrast,the same concentration of unmodified nanoparticles aggregated andseparated out of the host matrix.

FIGS. 6A and 6B illustrate fluorescence spectra of surface modifiednanoparticles and unmodified nanoparticles suspended in EX135 epoxyresin, respectively. For each of FIGS. 6A and 6B, four measurements wereperformed: one measurement of a blank sample with internal standard onlyand three measurements of the nanoparticles suspended in epoxy resin.The fluorescence quantum yield of the surface-modified nanoparticles(QY=60%) is greater than that of the unmodified nanoparticles (QY=58%).The quantum yield of the unmodified nanoparticles is decreased due toextensive aggregation and readbsorption effects.

Polystyrene-Compatible Nanoparticles.

Cadmium free quantum dot nanoparticles (CFQD) (InP/ZnS) (200 mg) withred emission at 608 nm was dispersed in toluene (1 mL) with isopropylmyristate (100 microliters). The mixture was heated at 50° C. for about1-2 minutes then slowly shook for 15 hours at room temperature. Atoluene solution (4 mL) of HMMM (Cymel 303) (400 mg), monomethoxypolyethylene oxide (CH₃O-PEG2000-OH) (400 mg), and salicylic acid (50mg) was added to the nanoparticle dispersion. The mixture was degasedand refluxed at 130° C. for 2 hours while stirring at 300 rpm with amagnetic stirrer. During the first hour a stream of nitrogen was passedthrough the flask to ensure the removal of volatile byproducts generatedby the reaction of HMMM with nucleophiles. The mixture was allowed tocool to room temperature and stored under inert gas. Thesurface-modified nanoparticles showed little or no loss in fluorescencequantum yield and no change in the emission peak or full width at halfmax (FWHM) value, compared to unmodified nanoparticles. When an aliquotof the modified dots was mixed with polystyrene or polystyrene copolymerresins (5% solids in toluene, e.g., styrene-ethylene/butylene-styrene orstyrene-ethylene/propylene-styrene (SEPS, SEBS, Kraton) the modifiednanoparticles dispersed very well in the host polystyrene resins andstayed dispersed even after removing the residual toluene. At the sameconcentration of nanoparticles, the unmodified crude nanoparticlesaggregated and separated out of the host resin. The film of thesurface-modified nanoparticle is uniform, whereas the film of theunmodified nanoparticle shows significant aggregation of nanoparticles.

3. Water-Compatible Nanoparticles.

Cadmium free quantum dot nanoparticles (CFQD) (InP/ZnS/ZnO) (200 mg)with red emission at 608 nm was dispersed in toluene (1 mL) withisopropyl myristate (100 microliters). The mixture was heated at 50° C.for about 1-2 minutes then slowly shook for 15 hours at roomtemperature. A toluene solution (4 mL) of HMMM (Cymel 303) (400 mg),monomethoxy polyethylene oxide (CH₃O-PEG2000-OH) (400 mg), and salicylicacid (50 mg) was added to the nanoparticle dispersion. The mixture wasdegased and refluxed at 140° C. for 4 hours while stirring at 300 rpmwith a magnetic stirrer. During the first hour a stream of nitrogen waspassed through the flask to ensure the removal of volatile byproductsgenerated by the reaction of HMMM with nucleophiles. The mixture wasallowed to cool to room temperature and stored under inert gas. Thesurface-modified nanoparticles showed little or no loss in fluorescencequantum yield and no change in the emission peak or full width at halfmax (FWHM) value, compared to unmodified nanoparticles.

An aliquot of the surface-modified nanoparticles was dried under vacuumand deionized water was added to the residue. The surface modifiednanoparticles dispersed well in the aqueous media and remained dispersedpermanently. In contrast, unmodified nanoparticles could not besuspended in the aqueous medium.

FIG. 7 shows fluorescence spectrum of the surface-modified nanoparticlesin water. Four measurements were performed: one measurement of a blanksample with internal standard only and three measurements of thenanoparticles suspended in epoxy resin. The fluorescence quantum yieldof the surface-modified nanoparticles is 47. It is noteworthy thattraditional methods for modifying nanoparticles to increase their watersolubility (e.g., ligand exchange with mercapto-functionalized watersoluble ligands) are ineffective under mild conditions to render thenanoparticles water soluble. Under harsher conditions, such as heat andsonication, the fraction that becomes water soluble has very low quantumyield (QY<20%). The instant method, in contrast, provides water solublenanoparticles with a high quantum yield.

Surface-modified nanoparticles prepared as in this example also dispersewell and remain permanently dispersed in other polar solvents, includingethanol, propanol, acetone, methylethylketone, butanol,tripropylmethylmethacrylate, or methylmethacrylate.

4. LED Stabilization and Brightness Enhancement.

Epoxy-compatible nanoparticles were prepared as described in Example 2.The epoxy-compatible nanoparticles were added to LED epoxy encapsulant(EX135). LEDs were prepared using the encapsulant and blue-emitting LEDchips. FIG. 8A illustrates emission curves of an LED incorporating thesurface-modified nanoparticles. Emission measurements were taken everydaily for one week and then weekly. For comparison, FIG. 8B illustratesemission curves of an LED incorporating unmodified nanoparticles. Theunmodified nanoparticles were first incorporated into acrylate beads,which were then encapsulated in epoxy. As expected, the emissionintensity of both LEDs decays over time as the LEDs degrade. However,the absolute emission intensity of the LED incorporating thesurface-modified nanoparticles is about twice the intensity of the LEDincorporating the unmodified nanoparticles.

FIGS. 9 A and 9 B show the percent efficacy a, percent emissionintensity b, and percent LED intensity c as a function of time for theLEDs incorporating the surface modified and the unmodifiednanoparticles, respectively. Percent efficacy is a measure of lightbrightness based on human eye sensitivity. Percent emission intensity isa measure of the intensity of the emission peak. Percent LED intensityis a measure of the blue LED chip intensity. The data illustrated inFIGS. 9 A and 9 B indicate that LED incorporating surface-modifiednanoparticles have comparable LED stability compared to the LEDincorporating unmodified nanoparticle embedded in highly crosslinkedpolymer beads. Incorporating the nanoparticles in highly crosslinkedbeads and then encapsulating the resulting beads in an LED encapsulant(e.g., EX135) is effective for maximizing the stability of thenanoparticles. However, LED devices using encapsulated beads suffer fromloss of brightness due to the beads' fabrication chemistry as well as tothe high rate of light scattering by the beads in the light path. TheLED using the surface-modified nanoparticles achieve comparable LEDstability to the encapsulated bead LED but has an absolute emissionintensity that is about twice the intensity of the LED incorporatingunmodified nanoparticles.

The foregoing description of preferred and other embodiments is notintended to limit or restrict the scope or applicability of theinventive concepts conceived of by the Applicants. It will beappreciated with the benefit of the present disclosure that featuresdescribed above in accordance with any embodiment or aspect of thedisclosed subject matter can be utilized, either alone or incombination, with any other described feature, in any other embodimentor aspect of the disclosed subject matter.

In exchange for disclosing the inventive concepts contained herein, theApplicants desire all patent rights afforded by the appended claims.Therefore, it is intended that the appended claims include allmodifications and alterations to the full extent that they come withinthe scope of the following claims or the equivalents thereof.

The invention claimed is:
 1. A method of functionalizing an InP/ZnSnanoparticle, comprising: associating, in a solvent, a ligandinteractive agent comprising a C₈-C₂₀ fatty acid with the nanoparticleto form a mixture comprising a ligand interactive agent-nanoparticleassociation complex; adding a linking/crosslinking agent comprising amelamine and a surface modifying ligand to the mixture; and reacting theligand interactive agent-nanoparticle association complex, thelinking/crosslinking agent and the surface modifying ligand to form afunctionalized nanoparticle having an outer surface comprising thesurface modifying ligand.
 2. The method of claim 1, further comprisingproviding capping ligands on the nanoparticle and wherein associatingthe ligand interactive agent with the nanoparticle comprisesintercalating the ligand interactive agent with the capping ligands. 3.The method of claim 1, wherein the linking/crosslinking agent provides abinding site for binding the surface modifying ligand to the ligandinteractive agent.
 4. The method of claim 1, wherein the surfacemodifying ligand is water soluble.
 5. The method of claim 1, wherein thesurface modifying ligand comprises a polyether.
 6. The method of claim1, wherein the surface modifying ligand is a monomethoxy polyethyleneglycol.
 7. The method of claim 1, wherein the surface modifying ligandcomprises a silicone.
 8. The method of claim 1, further comprisingadding a catalyst to the mixture.
 9. A composition comprising an InP/ZnSnanoparticle and a melamine compound.
 10. The composition of claim 9,wherein the nanoparticle further comprises: a ligand interactive agentcomprising a C₈-C₂₀ fatty acid and having a first portion associatedwith the surface nanoparticle and a second portion bound to the melaminecompound; and at least one surface modifying ligand bound to themelamine compound.
 11. The composition of claim 10, further comprising apolar solvent.
 12. The composition of claim 11, wherein the nanoparticlehas a quantum yield of greater than about 35%.
 13. The composition ofclaim 12, wherein the nanoparticle does not comprise cadmium.
 14. Thecomposition of claim 13, wherein the composition is an ink.
 15. Thecomposition of claim 10, further comprising capping ligands on thesurface of the nanoparticle, wherein the first portion of the ligandinteractive agent is intercalated with the capping ligands.
 16. Themethod of claim 8, wherein the catalyst is salicylic acid.
 17. Themethod of claim 8, wherein the catalyst is p-toluene sulfonic acid.